We propose to study the structure, chromosomal organization, and molecular genetics of the white locus of Drosophila melanogaster using molecular cloning procedures. Our primary experimental approach will be the analysis of differences in gene structure and expression between wild-type flies and flies mutant in white locus expression. Several classes of mutations at the white locus will be examined, including spontaneous mutations and their revertants, tandem duplications of all or part of the white locus which result in altered interactions with the zoste locus, and chromosomal rearrangements induced by X-rays. Alterations in the structure of the locus will be analyzed by the routine application of recombinant DNA technology (cloning and restriction mapping, whole genome blotting, and in situ hybridization). These studies will also be of value in understanding some of the mutagenic processes occurring in Drosophila. Changes in the expression of the locus produced by mutations at the white locus, as well as at other locations in the genome, will be monitored at the transcriptional level using sensitive methods involving the synthesis of single-stranded probes of high specific activity. By generating antibodies to potential white locus-specific protein determinants, it will be possible to examine modifications in the amount or molecular nature of proteins encoded by the locus produced by mutation. Such antibodies will also be of use in describing the tissue-specific and intracellular distributions of white locus products. The interaction between the loci white and zeste is dependent upon the somatic pairing of the chromosomal regions carrying the white locus, which in turn appears sensitive to the continuity of the regions between the white locus and a closely-adjacent constriction in the polytenized X chromosome. We propose to isolate clones spanning the constriction, in order to examine the biochemical features underlying the cytological appearance and genetic functions of these regions.